rev 51780 : imported patch syncknobs-00-base
rev 51781 : imported patch syncknobs-01-Knob_ReportSettings
rev 51782 : imported patch syncknobs-02-Knob_SpinBackOff
rev 51783 : imported patch syncknobs-03-BackOffMask
rev 51784 : imported patch syncknobs-04-Knob_ExitRelease
rev 51785 : imported patch syncknobs-05-Knob_InlineNotify
rev 51786 : imported patch syncknobs-06-Knob_Verbose
rev 51787 : imported patch syncknobs-07-Knob_VerifyInUse
rev 51788 : imported patch syncknobs-08-Knob_VerifyMatch
rev 51789 : imported patch syncknobs-09-Knob_SpinBase
rev 51790 : imported patch syncknobs-10-Knob_CASPenalty
rev 51791 : imported patch syncknobs-11-Knob_OXPenalty
rev 51792 : imported patch syncknobs-12-Knob_SpinSetSucc
rev 51793 : imported patch syncknobs-13-Knob_SpinEarly
rev 51794 : imported patch syncknobs-14-Knob_SuccEnabled
rev 51795 : imported patch syncknobs-15-Knob_SuccRestrict
rev 51796 : imported patch syncknobs-16-Knob_MaxSpinners
rev 51797 : imported patch syncknobs-17-Knob_SpinAfterFutile
rev 51798 : imported patch syncknobs-18-Knob_OState
rev 51799 : imported patch syncknobs-19-Knob_UsePause
rev 51800 : imported patch syncknobs-20-Knob_ExitPolicy
rev 51801 : imported patch syncknobs-21-Knob_ResetEvent
rev 51802 : imported patch syncknobs-22-Knob_FastHSSEC
rev 51803 : imported patch syncknobs-23-Knob_MoveNotifyee
rev 51804 : imported patch syncknobs-24-Knob_QMode

   1 /*
   2  * Copyright (c) 1998, 2018, Oracle and/or its affiliates. All rights reserved.
   3  * DO NOT ALTER OR REMOVE COPYRIGHT NOTICES OR THIS FILE HEADER.
   4  *
   5  * This code is free software; you can redistribute it and/or modify it
   6  * under the terms of the GNU General Public License version 2 only, as
   7  * published by the Free Software Foundation.
   8  *
   9  * This code is distributed in the hope that it will be useful, but WITHOUT
  10  * ANY WARRANTY; without even the implied warranty of MERCHANTABILITY or
  11  * FITNESS FOR A PARTICULAR PURPOSE.  See the GNU General Public License
  12  * version 2 for more details (a copy is included in the LICENSE file that
  13  * accompanied this code).
  14  *
  15  * You should have received a copy of the GNU General Public License version
  16  * 2 along with this work; if not, write to the Free Software Foundation,
  17  * Inc., 51 Franklin St, Fifth Floor, Boston, MA 02110-1301 USA.
  18  *
  19  * Please contact Oracle, 500 Oracle Parkway, Redwood Shores, CA 94065 USA
  20  * or visit www.oracle.com if you need additional information or have any
  21  * questions.
  22  *
  23  */
  24 
  25 #include "precompiled.hpp"
  26 #include "classfile/vmSymbols.hpp"
  27 #include "jfr/jfrEvents.hpp"
  28 #include "jfr/support/jfrThreadId.hpp"
  29 #include "memory/allocation.inline.hpp"
  30 #include "memory/resourceArea.hpp"
  31 #include "oops/markOop.hpp"
  32 #include "oops/oop.inline.hpp"
  33 #include "runtime/atomic.hpp"
  34 #include "runtime/handles.inline.hpp"
  35 #include "runtime/interfaceSupport.inline.hpp"
  36 #include "runtime/mutexLocker.hpp"
  37 #include "runtime/objectMonitor.hpp"
  38 #include "runtime/objectMonitor.inline.hpp"
  39 #include "runtime/orderAccess.hpp"
  40 #include "runtime/osThread.hpp"
  41 #include "runtime/safepointMechanism.inline.hpp"
  42 #include "runtime/sharedRuntime.hpp"
  43 #include "runtime/stubRoutines.hpp"
  44 #include "runtime/thread.inline.hpp"
  45 #include "services/threadService.hpp"
  46 #include "utilities/dtrace.hpp"
  47 #include "utilities/macros.hpp"
  48 #include "utilities/preserveException.hpp"
  49 #if INCLUDE_JFR
  50 #include "jfr/support/jfrFlush.hpp"
  51 #endif
  52 
  53 #ifdef DTRACE_ENABLED
  54 
  55 // Only bother with this argument setup if dtrace is available
  56 // TODO-FIXME: probes should not fire when caller is _blocked.  assert() accordingly.
  57 
  58 
  59 #define DTRACE_MONITOR_PROBE_COMMON(obj, thread)                           \
  60   char* bytes = NULL;                                                      \
  61   int len = 0;                                                             \
  62   jlong jtid = SharedRuntime::get_java_tid(thread);                        \
  63   Symbol* klassname = ((oop)obj)->klass()->name();                         \
  64   if (klassname != NULL) {                                                 \
  65     bytes = (char*)klassname->bytes();                                     \
  66     len = klassname->utf8_length();                                        \
  67   }
  68 
  69 #define DTRACE_MONITOR_WAIT_PROBE(monitor, obj, thread, millis)            \
  70   {                                                                        \
  71     if (DTraceMonitorProbes) {                                             \
  72       DTRACE_MONITOR_PROBE_COMMON(obj, thread);                            \
  73       HOTSPOT_MONITOR_WAIT(jtid,                                           \
  74                            (monitor), bytes, len, (millis));               \
  75     }                                                                      \
  76   }
  77 
  78 #define HOTSPOT_MONITOR_contended__enter HOTSPOT_MONITOR_CONTENDED_ENTER
  79 #define HOTSPOT_MONITOR_contended__entered HOTSPOT_MONITOR_CONTENDED_ENTERED
  80 #define HOTSPOT_MONITOR_contended__exit HOTSPOT_MONITOR_CONTENDED_EXIT
  81 #define HOTSPOT_MONITOR_notify HOTSPOT_MONITOR_NOTIFY
  82 #define HOTSPOT_MONITOR_notifyAll HOTSPOT_MONITOR_NOTIFYALL
  83 
  84 #define DTRACE_MONITOR_PROBE(probe, monitor, obj, thread)                  \
  85   {                                                                        \
  86     if (DTraceMonitorProbes) {                                             \
  87       DTRACE_MONITOR_PROBE_COMMON(obj, thread);                            \
  88       HOTSPOT_MONITOR_##probe(jtid,                                        \
  89                               (uintptr_t)(monitor), bytes, len);           \
  90     }                                                                      \
  91   }
  92 
  93 #else //  ndef DTRACE_ENABLED
  94 
  95 #define DTRACE_MONITOR_WAIT_PROBE(obj, thread, millis, mon)    {;}
  96 #define DTRACE_MONITOR_PROBE(probe, obj, thread, mon)          {;}
  97 
  98 #endif // ndef DTRACE_ENABLED
  99 
 100 // Tunables ...
 101 // The knob* variables are effectively final.  Once set they should
 102 // never be modified hence.  Consider using __read_mostly with GCC.
 103 





 104 int ObjectMonitor::Knob_SpinLimit    = 5000;    // derived by an external tool -
 105 










 106 static int Knob_Bonus               = 100;     // spin success bonus
 107 static int Knob_BonusB              = 100;     // spin success bonus
 108 static int Knob_Penalty             = 200;     // spin failure penalty
 109 static int Knob_Poverty             = 1000;

 110 static int Knob_FixedSpin           = 0;



 111 static int Knob_PreSpin             = 10;      // 20-100 likely better


 112 



 113 static volatile int InitDone        = 0;
 114 
 115 // -----------------------------------------------------------------------------
 116 // Theory of operations -- Monitors lists, thread residency, etc:
 117 //
 118 // * A thread acquires ownership of a monitor by successfully
 119 //   CAS()ing the _owner field from null to non-null.
 120 //
 121 // * Invariant: A thread appears on at most one monitor list --
 122 //   cxq, EntryList or WaitSet -- at any one time.
 123 //
 124 // * Contending threads "push" themselves onto the cxq with CAS
 125 //   and then spin/park.
 126 //
 127 // * After a contending thread eventually acquires the lock it must
 128 //   dequeue itself from either the EntryList or the cxq.
 129 //
 130 // * The exiting thread identifies and unparks an "heir presumptive"
 131 //   tentative successor thread on the EntryList.  Critically, the
 132 //   exiting thread doesn't unlink the successor thread from the EntryList.
 133 //   After having been unparked, the wakee will recontend for ownership of
 134 //   the monitor.   The successor (wakee) will either acquire the lock or
 135 //   re-park itself.
 136 //
 137 //   Succession is provided for by a policy of competitive handoff.
 138 //   The exiting thread does _not_ grant or pass ownership to the
 139 //   successor thread.  (This is also referred to as "handoff" succession").
 140 //   Instead the exiting thread releases ownership and possibly wakes
 141 //   a successor, so the successor can (re)compete for ownership of the lock.
 142 //   If the EntryList is empty but the cxq is populated the exiting
 143 //   thread will drain the cxq into the EntryList.  It does so by
 144 //   by detaching the cxq (installing null with CAS) and folding
 145 //   the threads from the cxq into the EntryList.  The EntryList is
 146 //   doubly linked, while the cxq is singly linked because of the
 147 //   CAS-based "push" used to enqueue recently arrived threads (RATs).
 148 //
 149 // * Concurrency invariants:
 150 //
 151 //   -- only the monitor owner may access or mutate the EntryList.
 152 //      The mutex property of the monitor itself protects the EntryList
 153 //      from concurrent interference.
 154 //   -- Only the monitor owner may detach the cxq.
 155 //
 156 // * The monitor entry list operations avoid locks, but strictly speaking
 157 //   they're not lock-free.  Enter is lock-free, exit is not.
 158 //   For a description of 'Methods and apparatus providing non-blocking access
 159 //   to a resource,' see U.S. Pat. No. 7844973.
 160 //
 161 // * The cxq can have multiple concurrent "pushers" but only one concurrent
 162 //   detaching thread.  This mechanism is immune from the ABA corruption.
 163 //   More precisely, the CAS-based "push" onto cxq is ABA-oblivious.
 164 //
 165 // * Taken together, the cxq and the EntryList constitute or form a
 166 //   single logical queue of threads stalled trying to acquire the lock.
 167 //   We use two distinct lists to improve the odds of a constant-time
 168 //   dequeue operation after acquisition (in the ::enter() epilogue) and
 169 //   to reduce heat on the list ends.  (c.f. Michael Scott's "2Q" algorithm).
 170 //   A key desideratum is to minimize queue & monitor metadata manipulation
 171 //   that occurs while holding the monitor lock -- that is, we want to
 172 //   minimize monitor lock holds times.  Note that even a small amount of
 173 //   fixed spinning will greatly reduce the # of enqueue-dequeue operations
 174 //   on EntryList|cxq.  That is, spinning relieves contention on the "inner"
 175 //   locks and monitor metadata.
 176 //
 177 //   Cxq points to the set of Recently Arrived Threads attempting entry.
 178 //   Because we push threads onto _cxq with CAS, the RATs must take the form of
 179 //   a singly-linked LIFO.  We drain _cxq into EntryList  at unlock-time when
 180 //   the unlocking thread notices that EntryList is null but _cxq is != null.
 181 //
 182 //   The EntryList is ordered by the prevailing queue discipline and
 183 //   can be organized in any convenient fashion, such as a doubly-linked list or
 184 //   a circular doubly-linked list.  Critically, we want insert and delete operations
 185 //   to operate in constant-time.  If we need a priority queue then something akin
 186 //   to Solaris' sleepq would work nicely.  Viz.,
 187 //   http://agg.eng/ws/on10_nightly/source/usr/src/uts/common/os/sleepq.c.
 188 //   Queue discipline is enforced at ::exit() time, when the unlocking thread
 189 //   drains the cxq into the EntryList, and orders or reorders the threads on the
 190 //   EntryList accordingly.
 191 //
 192 //   Barring "lock barging", this mechanism provides fair cyclic ordering,
 193 //   somewhat similar to an elevator-scan.
 194 //
 195 // * The monitor synchronization subsystem avoids the use of native
 196 //   synchronization primitives except for the narrow platform-specific
 197 //   park-unpark abstraction.  See the comments in os_solaris.cpp regarding
 198 //   the semantics of park-unpark.  Put another way, this monitor implementation
 199 //   depends only on atomic operations and park-unpark.  The monitor subsystem
 200 //   manages all RUNNING->BLOCKED and BLOCKED->READY transitions while the
 201 //   underlying OS manages the READY<->RUN transitions.
 202 //
 203 // * Waiting threads reside on the WaitSet list -- wait() puts
 204 //   the caller onto the WaitSet.
 205 //
 206 // * notify() or notifyAll() simply transfers threads from the WaitSet to
 207 //   either the EntryList or cxq.  Subsequent exit() operations will
 208 //   unpark the notifyee.  Unparking a notifee in notify() is inefficient -
 209 //   it's likely the notifyee would simply impale itself on the lock held
 210 //   by the notifier.
 211 //
 212 // * An interesting alternative is to encode cxq as (List,LockByte) where
 213 //   the LockByte is 0 iff the monitor is owned.  _owner is simply an auxiliary
 214 //   variable, like _recursions, in the scheme.  The threads or Events that form
 215 //   the list would have to be aligned in 256-byte addresses.  A thread would
 216 //   try to acquire the lock or enqueue itself with CAS, but exiting threads
 217 //   could use a 1-0 protocol and simply STB to set the LockByte to 0.
 218 //   Note that is is *not* word-tearing, but it does presume that full-word
 219 //   CAS operations are coherent with intermix with STB operations.  That's true
 220 //   on most common processors.
 221 //
 222 // * See also http://blogs.sun.com/dave
 223 
 224 
 225 void* ObjectMonitor::operator new (size_t size) throw() {
 226   return AllocateHeap(size, mtInternal);
 227 }
 228 void* ObjectMonitor::operator new[] (size_t size) throw() {
 229   return operator new (size);
 230 }
 231 void ObjectMonitor::operator delete(void* p) {
 232   FreeHeap(p);
 233 }
 234 void ObjectMonitor::operator delete[] (void *p) {
 235   operator delete(p);
 236 }
 237 
 238 // -----------------------------------------------------------------------------
 239 // Enter support
 240 
 241 void ObjectMonitor::enter(TRAPS) {
 242   // The following code is ordered to check the most common cases first
 243   // and to reduce RTS->RTO cache line upgrades on SPARC and IA32 processors.
 244   Thread * const Self = THREAD;
 245 
 246   void * cur = Atomic::cmpxchg(Self, &_owner, (void*)NULL);
 247   if (cur == NULL) {
 248     // Either ASSERT _recursions == 0 or explicitly set _recursions = 0.
 249     assert(_recursions == 0, "invariant");
 250     assert(_owner == Self, "invariant");
 251     return;
 252   }
 253 
 254   if (cur == Self) {
 255     // TODO-FIXME: check for integer overflow!  BUGID 6557169.
 256     _recursions++;
 257     return;
 258   }
 259 
 260   if (Self->is_lock_owned ((address)cur)) {
 261     assert(_recursions == 0, "internal state error");
 262     _recursions = 1;
 263     // Commute owner from a thread-specific on-stack BasicLockObject address to
 264     // a full-fledged "Thread *".
 265     _owner = Self;
 266     return;
 267   }
 268 
 269   // We've encountered genuine contention.
 270   assert(Self->_Stalled == 0, "invariant");
 271   Self->_Stalled = intptr_t(this);
 272 
 273   // Try one round of spinning *before* enqueueing Self
 274   // and before going through the awkward and expensive state
 275   // transitions.  The following spin is strictly optional ...
 276   // Note that if we acquire the monitor from an initial spin
 277   // we forgo posting JVMTI events and firing DTRACE probes.
 278   if (TrySpin(Self) > 0) {
 279     assert(_owner == Self, "invariant");
 280     assert(_recursions == 0, "invariant");
 281     assert(((oop)(object()))->mark() == markOopDesc::encode(this), "invariant");
 282     Self->_Stalled = 0;
 283     return;
 284   }
 285 
 286   assert(_owner != Self, "invariant");
 287   assert(_succ != Self, "invariant");
 288   assert(Self->is_Java_thread(), "invariant");
 289   JavaThread * jt = (JavaThread *) Self;
 290   assert(!SafepointSynchronize::is_at_safepoint(), "invariant");
 291   assert(jt->thread_state() != _thread_blocked, "invariant");
 292   assert(this->object() != NULL, "invariant");
 293   assert(_count >= 0, "invariant");
 294 
 295   // Prevent deflation at STW-time.  See deflate_idle_monitors() and is_busy().
 296   // Ensure the object-monitor relationship remains stable while there's contention.
 297   Atomic::inc(&_count);
 298 
 299   JFR_ONLY(JfrConditionalFlushWithStacktrace<EventJavaMonitorEnter> flush(jt);)
 300   EventJavaMonitorEnter event;
 301   if (event.should_commit()) {
 302     event.set_monitorClass(((oop)this->object())->klass());
 303     event.set_address((uintptr_t)(this->object_addr()));
 304   }
 305 
 306   { // Change java thread status to indicate blocked on monitor enter.
 307     JavaThreadBlockedOnMonitorEnterState jtbmes(jt, this);
 308 
 309     Self->set_current_pending_monitor(this);
 310 
 311     DTRACE_MONITOR_PROBE(contended__enter, this, object(), jt);
 312     if (JvmtiExport::should_post_monitor_contended_enter()) {
 313       JvmtiExport::post_monitor_contended_enter(jt, this);
 314 
 315       // The current thread does not yet own the monitor and does not
 316       // yet appear on any queues that would get it made the successor.
 317       // This means that the JVMTI_EVENT_MONITOR_CONTENDED_ENTER event
 318       // handler cannot accidentally consume an unpark() meant for the
 319       // ParkEvent associated with this ObjectMonitor.
 320     }
 321 
 322     OSThreadContendState osts(Self->osthread());
 323     ThreadBlockInVM tbivm(jt);
 324 
 325     // TODO-FIXME: change the following for(;;) loop to straight-line code.
 326     for (;;) {
 327       jt->set_suspend_equivalent();
 328       // cleared by handle_special_suspend_equivalent_condition()
 329       // or java_suspend_self()
 330 
 331       EnterI(THREAD);
 332 
 333       if (!ExitSuspendEquivalent(jt)) break;
 334 
 335       // We have acquired the contended monitor, but while we were
 336       // waiting another thread suspended us. We don't want to enter
 337       // the monitor while suspended because that would surprise the
 338       // thread that suspended us.
 339       //
 340       _recursions = 0;
 341       _succ = NULL;
 342       exit(false, Self);
 343 
 344       jt->java_suspend_self();
 345     }
 346     Self->set_current_pending_monitor(NULL);
 347 
 348     // We cleared the pending monitor info since we've just gotten past
 349     // the enter-check-for-suspend dance and we now own the monitor free
 350     // and clear, i.e., it is no longer pending. The ThreadBlockInVM
 351     // destructor can go to a safepoint at the end of this block. If we
 352     // do a thread dump during that safepoint, then this thread will show
 353     // as having "-locked" the monitor, but the OS and java.lang.Thread
 354     // states will still report that the thread is blocked trying to
 355     // acquire it.
 356   }
 357 
 358   Atomic::dec(&_count);
 359   assert(_count >= 0, "invariant");
 360   Self->_Stalled = 0;
 361 
 362   // Must either set _recursions = 0 or ASSERT _recursions == 0.
 363   assert(_recursions == 0, "invariant");
 364   assert(_owner == Self, "invariant");
 365   assert(_succ != Self, "invariant");
 366   assert(((oop)(object()))->mark() == markOopDesc::encode(this), "invariant");
 367 
 368   // The thread -- now the owner -- is back in vm mode.
 369   // Report the glorious news via TI,DTrace and jvmstat.
 370   // The probe effect is non-trivial.  All the reportage occurs
 371   // while we hold the monitor, increasing the length of the critical
 372   // section.  Amdahl's parallel speedup law comes vividly into play.
 373   //
 374   // Another option might be to aggregate the events (thread local or
 375   // per-monitor aggregation) and defer reporting until a more opportune
 376   // time -- such as next time some thread encounters contention but has
 377   // yet to acquire the lock.  While spinning that thread could
 378   // spinning we could increment JVMStat counters, etc.
 379 
 380   DTRACE_MONITOR_PROBE(contended__entered, this, object(), jt);
 381   if (JvmtiExport::should_post_monitor_contended_entered()) {
 382     JvmtiExport::post_monitor_contended_entered(jt, this);
 383 
 384     // The current thread already owns the monitor and is not going to
 385     // call park() for the remainder of the monitor enter protocol. So
 386     // it doesn't matter if the JVMTI_EVENT_MONITOR_CONTENDED_ENTERED
 387     // event handler consumed an unpark() issued by the thread that
 388     // just exited the monitor.
 389   }
 390   if (event.should_commit()) {
 391     event.set_previousOwner((uintptr_t)_previous_owner_tid);
 392     event.commit();
 393   }
 394   OM_PERFDATA_OP(ContendedLockAttempts, inc());
 395 }
 396 
 397 // Caveat: TryLock() is not necessarily serializing if it returns failure.
 398 // Callers must compensate as needed.
 399 
 400 int ObjectMonitor::TryLock(Thread * Self) {
 401   void * own = _owner;
 402   if (own != NULL) return 0;
 403   if (Atomic::replace_if_null(Self, &_owner)) {
 404     // Either guarantee _recursions == 0 or set _recursions = 0.
 405     assert(_recursions == 0, "invariant");
 406     assert(_owner == Self, "invariant");
 407     return 1;
 408   }
 409   // The lock had been free momentarily, but we lost the race to the lock.
 410   // Interference -- the CAS failed.
 411   // We can either return -1 or retry.
 412   // Retry doesn't make as much sense because the lock was just acquired.
 413   return -1;
 414 }
 415 
 416 #define MAX_RECHECK_INTERVAL 1000
 417 
 418 void ObjectMonitor::EnterI(TRAPS) {
 419   Thread * const Self = THREAD;
 420   assert(Self->is_Java_thread(), "invariant");
 421   assert(((JavaThread *) Self)->thread_state() == _thread_blocked, "invariant");
 422 
 423   // Try the lock - TATAS
 424   if (TryLock (Self) > 0) {
 425     assert(_succ != Self, "invariant");
 426     assert(_owner == Self, "invariant");
 427     assert(_Responsible != Self, "invariant");
 428     return;
 429   }
 430 
 431   DeferredInitialize();
 432 
 433   // We try one round of spinning *before* enqueueing Self.
 434   //
 435   // If the _owner is ready but OFFPROC we could use a YieldTo()
 436   // operation to donate the remainder of this thread's quantum
 437   // to the owner.  This has subtle but beneficial affinity
 438   // effects.
 439 
 440   if (TrySpin(Self) > 0) {
 441     assert(_owner == Self, "invariant");
 442     assert(_succ != Self, "invariant");
 443     assert(_Responsible != Self, "invariant");
 444     return;
 445   }
 446 
 447   // The Spin failed -- Enqueue and park the thread ...
 448   assert(_succ != Self, "invariant");
 449   assert(_owner != Self, "invariant");
 450   assert(_Responsible != Self, "invariant");
 451 
 452   // Enqueue "Self" on ObjectMonitor's _cxq.
 453   //
 454   // Node acts as a proxy for Self.
 455   // As an aside, if were to ever rewrite the synchronization code mostly
 456   // in Java, WaitNodes, ObjectMonitors, and Events would become 1st-class
 457   // Java objects.  This would avoid awkward lifecycle and liveness issues,
 458   // as well as eliminate a subset of ABA issues.
 459   // TODO: eliminate ObjectWaiter and enqueue either Threads or Events.
 460 
 461   ObjectWaiter node(Self);
 462   Self->_ParkEvent->reset();
 463   node._prev   = (ObjectWaiter *) 0xBAD;
 464   node.TState  = ObjectWaiter::TS_CXQ;
 465 
 466   // Push "Self" onto the front of the _cxq.
 467   // Once on cxq/EntryList, Self stays on-queue until it acquires the lock.
 468   // Note that spinning tends to reduce the rate at which threads
 469   // enqueue and dequeue on EntryList|cxq.
 470   ObjectWaiter * nxt;
 471   for (;;) {
 472     node._next = nxt = _cxq;
 473     if (Atomic::cmpxchg(&node, &_cxq, nxt) == nxt) break;
 474 
 475     // Interference - the CAS failed because _cxq changed.  Just retry.
 476     // As an optional optimization we retry the lock.
 477     if (TryLock (Self) > 0) {
 478       assert(_succ != Self, "invariant");
 479       assert(_owner == Self, "invariant");
 480       assert(_Responsible != Self, "invariant");
 481       return;
 482     }
 483   }
 484 
 485   // Check for cxq|EntryList edge transition to non-null.  This indicates
 486   // the onset of contention.  While contention persists exiting threads
 487   // will use a ST:MEMBAR:LD 1-1 exit protocol.  When contention abates exit
 488   // operations revert to the faster 1-0 mode.  This enter operation may interleave
 489   // (race) a concurrent 1-0 exit operation, resulting in stranding, so we
 490   // arrange for one of the contending thread to use a timed park() operations
 491   // to detect and recover from the race.  (Stranding is form of progress failure
 492   // where the monitor is unlocked but all the contending threads remain parked).
 493   // That is, at least one of the contended threads will periodically poll _owner.
 494   // One of the contending threads will become the designated "Responsible" thread.
 495   // The Responsible thread uses a timed park instead of a normal indefinite park
 496   // operation -- it periodically wakes and checks for and recovers from potential
 497   // strandings admitted by 1-0 exit operations.   We need at most one Responsible
 498   // thread per-monitor at any given moment.  Only threads on cxq|EntryList may
 499   // be responsible for a monitor.
 500   //
 501   // Currently, one of the contended threads takes on the added role of "Responsible".
 502   // A viable alternative would be to use a dedicated "stranding checker" thread
 503   // that periodically iterated over all the threads (or active monitors) and unparked
 504   // successors where there was risk of stranding.  This would help eliminate the
 505   // timer scalability issues we see on some platforms as we'd only have one thread
 506   // -- the checker -- parked on a timer.
 507 
 508   if (nxt == NULL && _EntryList == NULL) {
 509     // Try to assume the role of responsible thread for the monitor.
 510     // CONSIDER:  ST vs CAS vs { if (Responsible==null) Responsible=Self }
 511     Atomic::replace_if_null(Self, &_Responsible);
 512   }
 513 
 514   // The lock might have been released while this thread was occupied queueing
 515   // itself onto _cxq.  To close the race and avoid "stranding" and
 516   // progress-liveness failure we must resample-retry _owner before parking.
 517   // Note the Dekker/Lamport duality: ST cxq; MEMBAR; LD Owner.
 518   // In this case the ST-MEMBAR is accomplished with CAS().
 519   //
 520   // TODO: Defer all thread state transitions until park-time.
 521   // Since state transitions are heavy and inefficient we'd like
 522   // to defer the state transitions until absolutely necessary,
 523   // and in doing so avoid some transitions ...
 524 
 525   int nWakeups = 0;
 526   int recheckInterval = 1;
 527 
 528   for (;;) {
 529 
 530     if (TryLock(Self) > 0) break;
 531     assert(_owner != Self, "invariant");
 532 
 533     // park self
 534     if (_Responsible == Self) {
 535       Self->_ParkEvent->park((jlong) recheckInterval);
 536       // Increase the recheckInterval, but clamp the value.
 537       recheckInterval *= 8;
 538       if (recheckInterval > MAX_RECHECK_INTERVAL) {
 539         recheckInterval = MAX_RECHECK_INTERVAL;
 540       }
 541     } else {
 542       Self->_ParkEvent->park();
 543     }
 544 
 545     if (TryLock(Self) > 0) break;
 546 
 547     // The lock is still contested.
 548     // Keep a tally of the # of futile wakeups.
 549     // Note that the counter is not protected by a lock or updated by atomics.
 550     // That is by design - we trade "lossy" counters which are exposed to
 551     // races during updates for a lower probe effect.
 552 
 553     // This PerfData object can be used in parallel with a safepoint.
 554     // See the work around in PerfDataManager::destroy().
 555     OM_PERFDATA_OP(FutileWakeups, inc());
 556     ++nWakeups;
 557 
 558     // Assuming this is not a spurious wakeup we'll normally find _succ == Self.
 559     // We can defer clearing _succ until after the spin completes
 560     // TrySpin() must tolerate being called with _succ == Self.
 561     // Try yet another round of adaptive spinning.
 562     if (TrySpin(Self) > 0) break;
 563 
 564     // We can find that we were unpark()ed and redesignated _succ while
 565     // we were spinning.  That's harmless.  If we iterate and call park(),
 566     // park() will consume the event and return immediately and we'll
 567     // just spin again.  This pattern can repeat, leaving _succ to simply
 568     // spin on a CPU.


 569 




 570     if (_succ == Self) _succ = NULL;
 571 
 572     // Invariant: after clearing _succ a thread *must* retry _owner before parking.
 573     OrderAccess::fence();
 574   }
 575 
 576   // Egress :
 577   // Self has acquired the lock -- Unlink Self from the cxq or EntryList.
 578   // Normally we'll find Self on the EntryList .
 579   // From the perspective of the lock owner (this thread), the
 580   // EntryList is stable and cxq is prepend-only.
 581   // The head of cxq is volatile but the interior is stable.
 582   // In addition, Self.TState is stable.
 583 
 584   assert(_owner == Self, "invariant");
 585   assert(object() != NULL, "invariant");
 586   // I'd like to write:
 587   //   guarantee (((oop)(object()))->mark() == markOopDesc::encode(this), "invariant") ;
 588   // but as we're at a safepoint that's not safe.
 589 
 590   UnlinkAfterAcquire(Self, &node);
 591   if (_succ == Self) _succ = NULL;
 592 
 593   assert(_succ != Self, "invariant");
 594   if (_Responsible == Self) {
 595     _Responsible = NULL;
 596     OrderAccess::fence(); // Dekker pivot-point
 597 
 598     // We may leave threads on cxq|EntryList without a designated
 599     // "Responsible" thread.  This is benign.  When this thread subsequently
 600     // exits the monitor it can "see" such preexisting "old" threads --
 601     // threads that arrived on the cxq|EntryList before the fence, above --
 602     // by LDing cxq|EntryList.  Newly arrived threads -- that is, threads
 603     // that arrive on cxq after the ST:MEMBAR, above -- will set Responsible
 604     // non-null and elect a new "Responsible" timer thread.
 605     //
 606     // This thread executes:
 607     //    ST Responsible=null; MEMBAR    (in enter epilogue - here)
 608     //    LD cxq|EntryList               (in subsequent exit)
 609     //
 610     // Entering threads in the slow/contended path execute:
 611     //    ST cxq=nonnull; MEMBAR; LD Responsible (in enter prolog)
 612     //    The (ST cxq; MEMBAR) is accomplished with CAS().
 613     //
 614     // The MEMBAR, above, prevents the LD of cxq|EntryList in the subsequent
 615     // exit operation from floating above the ST Responsible=null.
 616   }
 617 
 618   // We've acquired ownership with CAS().
 619   // CAS is serializing -- it has MEMBAR/FENCE-equivalent semantics.
 620   // But since the CAS() this thread may have also stored into _succ,
 621   // EntryList, cxq or Responsible.  These meta-data updates must be
 622   // visible __before this thread subsequently drops the lock.
 623   // Consider what could occur if we didn't enforce this constraint --
 624   // STs to monitor meta-data and user-data could reorder with (become
 625   // visible after) the ST in exit that drops ownership of the lock.
 626   // Some other thread could then acquire the lock, but observe inconsistent
 627   // or old monitor meta-data and heap data.  That violates the JMM.
 628   // To that end, the 1-0 exit() operation must have at least STST|LDST
 629   // "release" barrier semantics.  Specifically, there must be at least a
 630   // STST|LDST barrier in exit() before the ST of null into _owner that drops
 631   // the lock.   The barrier ensures that changes to monitor meta-data and data
 632   // protected by the lock will be visible before we release the lock, and
 633   // therefore before some other thread (CPU) has a chance to acquire the lock.
 634   // See also: http://gee.cs.oswego.edu/dl/jmm/cookbook.html.
 635   //
 636   // Critically, any prior STs to _succ or EntryList must be visible before
 637   // the ST of null into _owner in the *subsequent* (following) corresponding
 638   // monitorexit.  Recall too, that in 1-0 mode monitorexit does not necessarily
 639   // execute a serializing instruction.
 640 
 641   return;
 642 }
 643 
 644 // ReenterI() is a specialized inline form of the latter half of the
 645 // contended slow-path from EnterI().  We use ReenterI() only for
 646 // monitor reentry in wait().
 647 //
 648 // In the future we should reconcile EnterI() and ReenterI().


 649 
 650 void ObjectMonitor::ReenterI(Thread * Self, ObjectWaiter * SelfNode) {
 651   assert(Self != NULL, "invariant");
 652   assert(SelfNode != NULL, "invariant");
 653   assert(SelfNode->_thread == Self, "invariant");
 654   assert(_waiters > 0, "invariant");
 655   assert(((oop)(object()))->mark() == markOopDesc::encode(this), "invariant");
 656   assert(((JavaThread *)Self)->thread_state() != _thread_blocked, "invariant");
 657   JavaThread * jt = (JavaThread *) Self;
 658 
 659   int nWakeups = 0;
 660   for (;;) {
 661     ObjectWaiter::TStates v = SelfNode->TState;
 662     guarantee(v == ObjectWaiter::TS_ENTER || v == ObjectWaiter::TS_CXQ, "invariant");
 663     assert(_owner != Self, "invariant");
 664 
 665     if (TryLock(Self) > 0) break;
 666     if (TrySpin(Self) > 0) break;
 667 
 668     // State transition wrappers around park() ...
 669     // ReenterI() wisely defers state transitions until
 670     // it's clear we must park the thread.
 671     {
 672       OSThreadContendState osts(Self->osthread());
 673       ThreadBlockInVM tbivm(jt);
 674 
 675       // cleared by handle_special_suspend_equivalent_condition()
 676       // or java_suspend_self()
 677       jt->set_suspend_equivalent();
 678       Self->_ParkEvent->park();
 679 
 680       // were we externally suspended while we were waiting?
 681       for (;;) {
 682         if (!ExitSuspendEquivalent(jt)) break;
 683         if (_succ == Self) { _succ = NULL; OrderAccess::fence(); }
 684         jt->java_suspend_self();
 685         jt->set_suspend_equivalent();
 686       }
 687     }
 688 
 689     // Try again, but just so we distinguish between futile wakeups and
 690     // successful wakeups.  The following test isn't algorithmically
 691     // necessary, but it helps us maintain sensible statistics.
 692     if (TryLock(Self) > 0) break;
 693 
 694     // The lock is still contested.
 695     // Keep a tally of the # of futile wakeups.
 696     // Note that the counter is not protected by a lock or updated by atomics.
 697     // That is by design - we trade "lossy" counters which are exposed to
 698     // races during updates for a lower probe effect.
 699     ++nWakeups;
 700 
 701     // Assuming this is not a spurious wakeup we'll normally
 702     // find that _succ == Self.
 703     if (_succ == Self) _succ = NULL;
 704 
 705     // Invariant: after clearing _succ a contending thread
 706     // *must* retry  _owner before parking.
 707     OrderAccess::fence();
 708 
 709     // This PerfData object can be used in parallel with a safepoint.
 710     // See the work around in PerfDataManager::destroy().
 711     OM_PERFDATA_OP(FutileWakeups, inc());
 712   }
 713 
 714   // Self has acquired the lock -- Unlink Self from the cxq or EntryList .
 715   // Normally we'll find Self on the EntryList.
 716   // Unlinking from the EntryList is constant-time and atomic-free.
 717   // From the perspective of the lock owner (this thread), the
 718   // EntryList is stable and cxq is prepend-only.
 719   // The head of cxq is volatile but the interior is stable.
 720   // In addition, Self.TState is stable.
 721 
 722   assert(_owner == Self, "invariant");
 723   assert(((oop)(object()))->mark() == markOopDesc::encode(this), "invariant");
 724   UnlinkAfterAcquire(Self, SelfNode);
 725   if (_succ == Self) _succ = NULL;
 726   assert(_succ != Self, "invariant");
 727   SelfNode->TState = ObjectWaiter::TS_RUN;
 728   OrderAccess::fence();      // see comments at the end of EnterI()
 729 }
 730 
 731 // By convention we unlink a contending thread from EntryList|cxq immediately
 732 // after the thread acquires the lock in ::enter().  Equally, we could defer
 733 // unlinking the thread until ::exit()-time.
 734 
 735 void ObjectMonitor::UnlinkAfterAcquire(Thread *Self, ObjectWaiter *SelfNode) {
 736   assert(_owner == Self, "invariant");
 737   assert(SelfNode->_thread == Self, "invariant");
 738 
 739   if (SelfNode->TState == ObjectWaiter::TS_ENTER) {
 740     // Normal case: remove Self from the DLL EntryList .
 741     // This is a constant-time operation.
 742     ObjectWaiter * nxt = SelfNode->_next;
 743     ObjectWaiter * prv = SelfNode->_prev;
 744     if (nxt != NULL) nxt->_prev = prv;
 745     if (prv != NULL) prv->_next = nxt;
 746     if (SelfNode == _EntryList) _EntryList = nxt;
 747     assert(nxt == NULL || nxt->TState == ObjectWaiter::TS_ENTER, "invariant");
 748     assert(prv == NULL || prv->TState == ObjectWaiter::TS_ENTER, "invariant");
 749   } else {
 750     assert(SelfNode->TState == ObjectWaiter::TS_CXQ, "invariant");
 751     // Inopportune interleaving -- Self is still on the cxq.
 752     // This usually means the enqueue of self raced an exiting thread.
 753     // Normally we'll find Self near the front of the cxq, so
 754     // dequeueing is typically fast.  If needbe we can accelerate
 755     // this with some MCS/CHL-like bidirectional list hints and advisory
 756     // back-links so dequeueing from the interior will normally operate
 757     // in constant-time.
 758     // Dequeue Self from either the head (with CAS) or from the interior
 759     // with a linear-time scan and normal non-atomic memory operations.
 760     // CONSIDER: if Self is on the cxq then simply drain cxq into EntryList
 761     // and then unlink Self from EntryList.  We have to drain eventually,
 762     // so it might as well be now.
 763 
 764     ObjectWaiter * v = _cxq;
 765     assert(v != NULL, "invariant");
 766     if (v != SelfNode || Atomic::cmpxchg(SelfNode->_next, &_cxq, v) != v) {
 767       // The CAS above can fail from interference IFF a "RAT" arrived.
 768       // In that case Self must be in the interior and can no longer be
 769       // at the head of cxq.
 770       if (v == SelfNode) {
 771         assert(_cxq != v, "invariant");
 772         v = _cxq;          // CAS above failed - start scan at head of list
 773       }
 774       ObjectWaiter * p;
 775       ObjectWaiter * q = NULL;
 776       for (p = v; p != NULL && p != SelfNode; p = p->_next) {
 777         q = p;
 778         assert(p->TState == ObjectWaiter::TS_CXQ, "invariant");
 779       }
 780       assert(v != SelfNode, "invariant");
 781       assert(p == SelfNode, "Node not found on cxq");
 782       assert(p != _cxq, "invariant");
 783       assert(q != NULL, "invariant");
 784       assert(q->_next == p, "invariant");
 785       q->_next = p->_next;
 786     }
 787   }
 788 
 789 #ifdef ASSERT
 790   // Diagnostic hygiene ...
 791   SelfNode->_prev  = (ObjectWaiter *) 0xBAD;
 792   SelfNode->_next  = (ObjectWaiter *) 0xBAD;
 793   SelfNode->TState = ObjectWaiter::TS_RUN;
 794 #endif
 795 }
 796 
 797 // -----------------------------------------------------------------------------
 798 // Exit support
 799 //
 800 // exit()
 801 // ~~~~~~
 802 // Note that the collector can't reclaim the objectMonitor or deflate
 803 // the object out from underneath the thread calling ::exit() as the
 804 // thread calling ::exit() never transitions to a stable state.
 805 // This inhibits GC, which in turn inhibits asynchronous (and
 806 // inopportune) reclamation of "this".
 807 //
 808 // We'd like to assert that: (THREAD->thread_state() != _thread_blocked) ;
 809 // There's one exception to the claim above, however.  EnterI() can call
 810 // exit() to drop a lock if the acquirer has been externally suspended.
 811 // In that case exit() is called with _thread_state as _thread_blocked,
 812 // but the monitor's _count field is > 0, which inhibits reclamation.
 813 //
 814 // 1-0 exit
 815 // ~~~~~~~~
 816 // ::exit() uses a canonical 1-1 idiom with a MEMBAR although some of
 817 // the fast-path operators have been optimized so the common ::exit()
 818 // operation is 1-0, e.g., see macroAssembler_x86.cpp: fast_unlock().
 819 // The code emitted by fast_unlock() elides the usual MEMBAR.  This
 820 // greatly improves latency -- MEMBAR and CAS having considerable local
 821 // latency on modern processors -- but at the cost of "stranding".  Absent the
 822 // MEMBAR, a thread in fast_unlock() can race a thread in the slow
 823 // ::enter() path, resulting in the entering thread being stranding
 824 // and a progress-liveness failure.   Stranding is extremely rare.
 825 // We use timers (timed park operations) & periodic polling to detect
 826 // and recover from stranding.  Potentially stranded threads periodically
 827 // wake up and poll the lock.  See the usage of the _Responsible variable.
 828 //
 829 // The CAS() in enter provides for safety and exclusion, while the CAS or
 830 // MEMBAR in exit provides for progress and avoids stranding.  1-0 locking
 831 // eliminates the CAS/MEMBAR from the exit path, but it admits stranding.
 832 // We detect and recover from stranding with timers.
 833 //
 834 // If a thread transiently strands it'll park until (a) another
 835 // thread acquires the lock and then drops the lock, at which time the
 836 // exiting thread will notice and unpark the stranded thread, or, (b)
 837 // the timer expires.  If the lock is high traffic then the stranding latency
 838 // will be low due to (a).  If the lock is low traffic then the odds of
 839 // stranding are lower, although the worst-case stranding latency
 840 // is longer.  Critically, we don't want to put excessive load in the
 841 // platform's timer subsystem.  We want to minimize both the timer injection
 842 // rate (timers created/sec) as well as the number of timers active at
 843 // any one time.  (more precisely, we want to minimize timer-seconds, which is
 844 // the integral of the # of active timers at any instant over time).
 845 // Both impinge on OS scalability.  Given that, at most one thread parked on
 846 // a monitor will use a timer.
 847 //
 848 // There is also the risk of a futile wake-up. If we drop the lock
 849 // another thread can reacquire the lock immediately, and we can
 850 // then wake a thread unnecessarily. This is benign, and we've
 851 // structured the code so the windows are short and the frequency
 852 // of such futile wakups is low.
 853 
 854 void ObjectMonitor::exit(bool not_suspended, TRAPS) {
 855   Thread * const Self = THREAD;
 856   if (THREAD != _owner) {
 857     if (THREAD->is_lock_owned((address) _owner)) {
 858       // Transmute _owner from a BasicLock pointer to a Thread address.
 859       // We don't need to hold _mutex for this transition.
 860       // Non-null to Non-null is safe as long as all readers can
 861       // tolerate either flavor.
 862       assert(_recursions == 0, "invariant");
 863       _owner = THREAD;
 864       _recursions = 0;
 865     } else {
 866       // Apparent unbalanced locking ...
 867       // Naively we'd like to throw IllegalMonitorStateException.
 868       // As a practical matter we can neither allocate nor throw an
 869       // exception as ::exit() can be called from leaf routines.
 870       // see x86_32.ad Fast_Unlock() and the I1 and I2 properties.
 871       // Upon deeper reflection, however, in a properly run JVM the only
 872       // way we should encounter this situation is in the presence of
 873       // unbalanced JNI locking. TODO: CheckJNICalls.
 874       // See also: CR4414101
 875       assert(false, "Non-balanced monitor enter/exit! Likely JNI locking");
 876       return;
 877     }
 878   }
 879 
 880   if (_recursions != 0) {
 881     _recursions--;        // this is simple recursive enter
 882     return;
 883   }
 884 
 885   // Invariant: after setting Responsible=null an thread must execute
 886   // a MEMBAR or other serializing instruction before fetching EntryList|cxq.
 887   _Responsible = NULL;
 888 
 889 #if INCLUDE_JFR
 890   // get the owner's thread id for the MonitorEnter event
 891   // if it is enabled and the thread isn't suspended
 892   if (not_suspended && EventJavaMonitorEnter::is_enabled()) {
 893     _previous_owner_tid = JFR_THREAD_ID(Self);
 894   }
 895 #endif
 896 
 897   for (;;) {
 898     assert(THREAD == _owner, "invariant");
 899 

 900     // release semantics: prior loads and stores from within the critical section
 901     // must not float (reorder) past the following store that drops the lock.
 902     // On SPARC that requires MEMBAR #loadstore|#storestore.
 903     // But of course in TSO #loadstore|#storestore is not required.
 904     // I'd like to write one of the following:
 905     // A.  OrderAccess::release() ; _owner = NULL
 906     // B.  OrderAccess::loadstore(); OrderAccess::storestore(); _owner = NULL;
 907     // Unfortunately OrderAccess::release() and OrderAccess::loadstore() both
 908     // store into a _dummy variable.  That store is not needed, but can result
 909     // in massive wasteful coherency traffic on classic SMP systems.
 910     // Instead, I use release_store(), which is implemented as just a simple
 911     // ST on x64, x86 and SPARC.
 912     OrderAccess::release_store(&_owner, (void*)NULL);   // drop the lock
 913     OrderAccess::storeload();                        // See if we need to wake a successor
 914     if ((intptr_t(_EntryList)|intptr_t(_cxq)) == 0 || _succ != NULL) {
 915       return;
 916     }
 917     // Other threads are blocked trying to acquire the lock.
 918 
 919     // Normally the exiting thread is responsible for ensuring succession,
 920     // but if other successors are ready or other entering threads are spinning
 921     // then this thread can simply store NULL into _owner and exit without
 922     // waking a successor.  The existence of spinners or ready successors
 923     // guarantees proper succession (liveness).  Responsibility passes to the
 924     // ready or running successors.  The exiting thread delegates the duty.
 925     // More precisely, if a successor already exists this thread is absolved
 926     // of the responsibility of waking (unparking) one.
 927     //
 928     // The _succ variable is critical to reducing futile wakeup frequency.
 929     // _succ identifies the "heir presumptive" thread that has been made
 930     // ready (unparked) but that has not yet run.  We need only one such
 931     // successor thread to guarantee progress.
 932     // See http://www.usenix.org/events/jvm01/full_papers/dice/dice.pdf
 933     // section 3.3 "Futile Wakeup Throttling" for details.
 934     //
 935     // Note that spinners in Enter() also set _succ non-null.
 936     // In the current implementation spinners opportunistically set
 937     // _succ so that exiting threads might avoid waking a successor.
 938     // Another less appealing alternative would be for the exiting thread
 939     // to drop the lock and then spin briefly to see if a spinner managed
 940     // to acquire the lock.  If so, the exiting thread could exit
 941     // immediately without waking a successor, otherwise the exiting
 942     // thread would need to dequeue and wake a successor.
 943     // (Note that we'd need to make the post-drop spin short, but no
 944     // shorter than the worst-case round-trip cache-line migration time.
 945     // The dropped lock needs to become visible to the spinner, and then
 946     // the acquisition of the lock by the spinner must become visible to
 947     // the exiting thread).
 948 
 949     // It appears that an heir-presumptive (successor) must be made ready.
 950     // Only the current lock owner can manipulate the EntryList or
 951     // drain _cxq, so we need to reacquire the lock.  If we fail
 952     // to reacquire the lock the responsibility for ensuring succession
 953     // falls to the new owner.
 954     //
 955     if (!Atomic::replace_if_null(THREAD, &_owner)) {
 956       return;
 957     }

























 958 
 959     guarantee(_owner == THREAD, "invariant");
 960 
 961     ObjectWaiter * w = NULL;























































































 962 
 963     w = _EntryList;
 964     if (w != NULL) {
 965       // I'd like to write: guarantee (w->_thread != Self).
 966       // But in practice an exiting thread may find itself on the EntryList.
 967       // Let's say thread T1 calls O.wait().  Wait() enqueues T1 on O's waitset and
 968       // then calls exit().  Exit release the lock by setting O._owner to NULL.
 969       // Let's say T1 then stalls.  T2 acquires O and calls O.notify().  The
 970       // notify() operation moves T1 from O's waitset to O's EntryList. T2 then
 971       // release the lock "O".  T2 resumes immediately after the ST of null into
 972       // _owner, above.  T2 notices that the EntryList is populated, so it
 973       // reacquires the lock and then finds itself on the EntryList.
 974       // Given all that, we have to tolerate the circumstance where "w" is
 975       // associated with Self.
 976       assert(w->TState == ObjectWaiter::TS_ENTER, "invariant");
 977       ExitEpilog(Self, w);
 978       return;
 979     }
 980 
 981     // If we find that both _cxq and EntryList are null then just
 982     // re-run the exit protocol from the top.
 983     w = _cxq;
 984     if (w == NULL) continue;
 985 
 986     // Drain _cxq into EntryList - bulk transfer.
 987     // First, detach _cxq.
 988     // The following loop is tantamount to: w = swap(&cxq, NULL)
 989     for (;;) {
 990       assert(w != NULL, "Invariant");
 991       ObjectWaiter * u = Atomic::cmpxchg((ObjectWaiter*)NULL, &_cxq, w);
 992       if (u == w) break;
 993       w = u;
 994     }
 995 
 996     assert(w != NULL, "invariant");
 997     assert(_EntryList == NULL, "invariant");
 998 
 999     // Convert the LIFO SLL anchored by _cxq into a DLL.
1000     // The list reorganization step operates in O(LENGTH(w)) time.
1001     // It's critical that this step operate quickly as
1002     // "Self" still holds the outer-lock, restricting parallelism
1003     // and effectively lengthening the critical section.
1004     // Invariant: s chases t chases u.
1005     // TODO-FIXME: consider changing EntryList from a DLL to a CDLL so
1006     // we have faster access to the tail.
1007 



















1008     _EntryList = w;
1009     ObjectWaiter * q = NULL;
1010     ObjectWaiter * p;
1011     for (p = w; p != NULL; p = p->_next) {
1012       guarantee(p->TState == ObjectWaiter::TS_CXQ, "Invariant");
1013       p->TState = ObjectWaiter::TS_ENTER;
1014       p->_prev = q;
1015       q = p;
1016     }

1017 
1018     // In 1-0 mode we need: ST EntryList; MEMBAR #storestore; ST _owner = NULL
1019     // The MEMBAR is satisfied by the release_store() operation in ExitEpilog().
1020 
1021     // See if we can abdicate to a spinner instead of waking a thread.
1022     // A primary goal of the implementation is to reduce the
1023     // context-switch rate.
1024     if (_succ != NULL) continue;
1025 
1026     w = _EntryList;
1027     if (w != NULL) {
1028       guarantee(w->TState == ObjectWaiter::TS_ENTER, "invariant");
1029       ExitEpilog(Self, w);
1030       return;
1031     }
1032   }
1033 }
1034 
1035 // ExitSuspendEquivalent:
1036 // A faster alternate to handle_special_suspend_equivalent_condition()
1037 //
1038 // handle_special_suspend_equivalent_condition() unconditionally
1039 // acquires the SR_lock.  On some platforms uncontended MutexLocker()
1040 // operations have high latency.  Note that in ::enter() we call HSSEC
1041 // while holding the monitor, so we effectively lengthen the critical sections.
1042 //
1043 // There are a number of possible solutions:
1044 //
1045 // A.  To ameliorate the problem we might also defer state transitions
1046 //     to as late as possible -- just prior to parking.
1047 //     Given that, we'd call HSSEC after having returned from park(),
1048 //     but before attempting to acquire the monitor.  This is only a
1049 //     partial solution.  It avoids calling HSSEC while holding the
1050 //     monitor (good), but it still increases successor reacquisition latency --
1051 //     the interval between unparking a successor and the time the successor
1052 //     resumes and retries the lock.  See ReenterI(), which defers state transitions.
1053 //     If we use this technique we can also avoid EnterI()-exit() loop
1054 //     in ::enter() where we iteratively drop the lock and then attempt
1055 //     to reacquire it after suspending.
1056 //
1057 // B.  In the future we might fold all the suspend bits into a
1058 //     composite per-thread suspend flag and then update it with CAS().
1059 //     Alternately, a Dekker-like mechanism with multiple variables
1060 //     would suffice:
1061 //       ST Self->_suspend_equivalent = false
1062 //       MEMBAR
1063 //       LD Self_>_suspend_flags





1064 
1065 bool ObjectMonitor::ExitSuspendEquivalent(JavaThread * jSelf) {









1066   return jSelf->handle_special_suspend_equivalent_condition();
1067 }
1068 
1069 
1070 void ObjectMonitor::ExitEpilog(Thread * Self, ObjectWaiter * Wakee) {
1071   assert(_owner == Self, "invariant");
1072 
1073   // Exit protocol:
1074   // 1. ST _succ = wakee
1075   // 2. membar #loadstore|#storestore;
1076   // 2. ST _owner = NULL
1077   // 3. unpark(wakee)
1078 
1079   _succ = Wakee->_thread;
1080   ParkEvent * Trigger = Wakee->_event;
1081 
1082   // Hygiene -- once we've set _owner = NULL we can't safely dereference Wakee again.
1083   // The thread associated with Wakee may have grabbed the lock and "Wakee" may be
1084   // out-of-scope (non-extant).
1085   Wakee  = NULL;
1086 
1087   // Drop the lock
1088   OrderAccess::release_store(&_owner, (void*)NULL);
1089   OrderAccess::fence();                               // ST _owner vs LD in unpark()
1090 
1091   DTRACE_MONITOR_PROBE(contended__exit, this, object(), Self);
1092   Trigger->unpark();
1093 
1094   // Maintain stats and report events to JVMTI
1095   OM_PERFDATA_OP(Parks, inc());
1096 }
1097 
1098 
1099 // -----------------------------------------------------------------------------
1100 // Class Loader deadlock handling.
1101 //
1102 // complete_exit exits a lock returning recursion count
1103 // complete_exit/reenter operate as a wait without waiting
1104 // complete_exit requires an inflated monitor
1105 // The _owner field is not always the Thread addr even with an
1106 // inflated monitor, e.g. the monitor can be inflated by a non-owning
1107 // thread due to contention.
1108 intptr_t ObjectMonitor::complete_exit(TRAPS) {
1109   Thread * const Self = THREAD;
1110   assert(Self->is_Java_thread(), "Must be Java thread!");
1111   JavaThread *jt = (JavaThread *)THREAD;
1112 
1113   DeferredInitialize();
1114 
1115   if (THREAD != _owner) {
1116     if (THREAD->is_lock_owned ((address)_owner)) {
1117       assert(_recursions == 0, "internal state error");
1118       _owner = THREAD;   // Convert from basiclock addr to Thread addr
1119       _recursions = 0;
1120     }
1121   }
1122 
1123   guarantee(Self == _owner, "complete_exit not owner");
1124   intptr_t save = _recursions; // record the old recursion count
1125   _recursions = 0;        // set the recursion level to be 0
1126   exit(true, Self);           // exit the monitor
1127   guarantee(_owner != Self, "invariant");
1128   return save;
1129 }
1130 
1131 // reenter() enters a lock and sets recursion count
1132 // complete_exit/reenter operate as a wait without waiting
1133 void ObjectMonitor::reenter(intptr_t recursions, TRAPS) {
1134   Thread * const Self = THREAD;
1135   assert(Self->is_Java_thread(), "Must be Java thread!");
1136   JavaThread *jt = (JavaThread *)THREAD;
1137 
1138   guarantee(_owner != Self, "reenter already owner");
1139   enter(THREAD);       // enter the monitor
1140   guarantee(_recursions == 0, "reenter recursion");
1141   _recursions = recursions;
1142   return;
1143 }
1144 
1145 
1146 // -----------------------------------------------------------------------------
1147 // A macro is used below because there may already be a pending
1148 // exception which should not abort the execution of the routines
1149 // which use this (which is why we don't put this into check_slow and
1150 // call it with a CHECK argument).
1151 
1152 #define CHECK_OWNER()                                                       \
1153   do {                                                                      \
1154     if (THREAD != _owner) {                                                 \
1155       if (THREAD->is_lock_owned((address) _owner)) {                        \
1156         _owner = THREAD;  /* Convert from basiclock addr to Thread addr */  \
1157         _recursions = 0;                                                    \
1158       } else {                                                              \
1159         THROW(vmSymbols::java_lang_IllegalMonitorStateException());         \
1160       }                                                                     \
1161     }                                                                       \
1162   } while (false)
1163 
1164 // check_slow() is a misnomer.  It's called to simply to throw an IMSX exception.
1165 // TODO-FIXME: remove check_slow() -- it's likely dead.
1166 
1167 void ObjectMonitor::check_slow(TRAPS) {
1168   assert(THREAD != _owner && !THREAD->is_lock_owned((address) _owner), "must not be owner");
1169   THROW_MSG(vmSymbols::java_lang_IllegalMonitorStateException(), "current thread not owner");
1170 }
1171 






1172 static void post_monitor_wait_event(EventJavaMonitorWait* event,
1173                                     ObjectMonitor* monitor,
1174                                     jlong notifier_tid,
1175                                     jlong timeout,
1176                                     bool timedout) {
1177   assert(event != NULL, "invariant");
1178   assert(monitor != NULL, "invariant");
1179   event->set_monitorClass(((oop)monitor->object())->klass());
1180   event->set_timeout(timeout);
1181   event->set_address((uintptr_t)monitor->object_addr());
1182   event->set_notifier(notifier_tid);
1183   event->set_timedOut(timedout);
1184   event->commit();
1185 }
1186 
1187 // -----------------------------------------------------------------------------
1188 // Wait/Notify/NotifyAll
1189 //
1190 // Note: a subset of changes to ObjectMonitor::wait()
1191 // will need to be replicated in complete_exit
1192 void ObjectMonitor::wait(jlong millis, bool interruptible, TRAPS) {
1193   Thread * const Self = THREAD;
1194   assert(Self->is_Java_thread(), "Must be Java thread!");
1195   JavaThread *jt = (JavaThread *)THREAD;
1196 
1197   DeferredInitialize();
1198 
1199   // Throw IMSX or IEX.
1200   CHECK_OWNER();
1201 
1202   EventJavaMonitorWait event;
1203 
1204   // check for a pending interrupt
1205   if (interruptible && Thread::is_interrupted(Self, true) && !HAS_PENDING_EXCEPTION) {
1206     // post monitor waited event.  Note that this is past-tense, we are done waiting.
1207     if (JvmtiExport::should_post_monitor_waited()) {
1208       // Note: 'false' parameter is passed here because the
1209       // wait was not timed out due to thread interrupt.
1210       JvmtiExport::post_monitor_waited(jt, this, false);
1211 
1212       // In this short circuit of the monitor wait protocol, the
1213       // current thread never drops ownership of the monitor and
1214       // never gets added to the wait queue so the current thread
1215       // cannot be made the successor. This means that the
1216       // JVMTI_EVENT_MONITOR_WAITED event handler cannot accidentally
1217       // consume an unpark() meant for the ParkEvent associated with
1218       // this ObjectMonitor.
1219     }
1220     if (event.should_commit()) {
1221       post_monitor_wait_event(&event, this, 0, millis, false);
1222     }
1223     THROW(vmSymbols::java_lang_InterruptedException());
1224     return;
1225   }
1226 
1227   assert(Self->_Stalled == 0, "invariant");
1228   Self->_Stalled = intptr_t(this);
1229   jt->set_current_waiting_monitor(this);
1230 
1231   // create a node to be put into the queue
1232   // Critically, after we reset() the event but prior to park(), we must check
1233   // for a pending interrupt.
1234   ObjectWaiter node(Self);
1235   node.TState = ObjectWaiter::TS_WAIT;
1236   Self->_ParkEvent->reset();
1237   OrderAccess::fence();          // ST into Event; membar ; LD interrupted-flag
1238 
1239   // Enter the waiting queue, which is a circular doubly linked list in this case
1240   // but it could be a priority queue or any data structure.
1241   // _WaitSetLock protects the wait queue.  Normally the wait queue is accessed only
1242   // by the the owner of the monitor *except* in the case where park()
1243   // returns because of a timeout of interrupt.  Contention is exceptionally rare
1244   // so we use a simple spin-lock instead of a heavier-weight blocking lock.
1245 
1246   Thread::SpinAcquire(&_WaitSetLock, "WaitSet - add");
1247   AddWaiter(&node);
1248   Thread::SpinRelease(&_WaitSetLock);
1249 
1250   _Responsible = NULL;
1251 
1252   intptr_t save = _recursions; // record the old recursion count
1253   _waiters++;                  // increment the number of waiters
1254   _recursions = 0;             // set the recursion level to be 1
1255   exit(true, Self);                    // exit the monitor
1256   guarantee(_owner != Self, "invariant");
1257 
1258   // The thread is on the WaitSet list - now park() it.
1259   // On MP systems it's conceivable that a brief spin before we park
1260   // could be profitable.
1261   //
1262   // TODO-FIXME: change the following logic to a loop of the form
1263   //   while (!timeout && !interrupted && _notified == 0) park()
1264 
1265   int ret = OS_OK;
1266   int WasNotified = 0;
1267   { // State transition wrappers
1268     OSThread* osthread = Self->osthread();
1269     OSThreadWaitState osts(osthread, true);
1270     {
1271       ThreadBlockInVM tbivm(jt);
1272       // Thread is in thread_blocked state and oop access is unsafe.
1273       jt->set_suspend_equivalent();
1274 
1275       if (interruptible && (Thread::is_interrupted(THREAD, false) || HAS_PENDING_EXCEPTION)) {
1276         // Intentionally empty
1277       } else if (node._notified == 0) {
1278         if (millis <= 0) {
1279           Self->_ParkEvent->park();
1280         } else {
1281           ret = Self->_ParkEvent->park(millis);
1282         }
1283       }
1284 
1285       // were we externally suspended while we were waiting?
1286       if (ExitSuspendEquivalent (jt)) {
1287         // TODO-FIXME: add -- if succ == Self then succ = null.
1288         jt->java_suspend_self();
1289       }
1290 
1291     } // Exit thread safepoint: transition _thread_blocked -> _thread_in_vm
1292 
1293     // Node may be on the WaitSet, the EntryList (or cxq), or in transition
1294     // from the WaitSet to the EntryList.
1295     // See if we need to remove Node from the WaitSet.
1296     // We use double-checked locking to avoid grabbing _WaitSetLock
1297     // if the thread is not on the wait queue.
1298     //
1299     // Note that we don't need a fence before the fetch of TState.
1300     // In the worst case we'll fetch a old-stale value of TS_WAIT previously
1301     // written by the is thread. (perhaps the fetch might even be satisfied
1302     // by a look-aside into the processor's own store buffer, although given
1303     // the length of the code path between the prior ST and this load that's
1304     // highly unlikely).  If the following LD fetches a stale TS_WAIT value
1305     // then we'll acquire the lock and then re-fetch a fresh TState value.
1306     // That is, we fail toward safety.
1307 
1308     if (node.TState == ObjectWaiter::TS_WAIT) {
1309       Thread::SpinAcquire(&_WaitSetLock, "WaitSet - unlink");
1310       if (node.TState == ObjectWaiter::TS_WAIT) {
1311         DequeueSpecificWaiter(&node);       // unlink from WaitSet
1312         assert(node._notified == 0, "invariant");
1313         node.TState = ObjectWaiter::TS_RUN;
1314       }
1315       Thread::SpinRelease(&_WaitSetLock);
1316     }
1317 
1318     // The thread is now either on off-list (TS_RUN),
1319     // on the EntryList (TS_ENTER), or on the cxq (TS_CXQ).
1320     // The Node's TState variable is stable from the perspective of this thread.
1321     // No other threads will asynchronously modify TState.
1322     guarantee(node.TState != ObjectWaiter::TS_WAIT, "invariant");
1323     OrderAccess::loadload();
1324     if (_succ == Self) _succ = NULL;
1325     WasNotified = node._notified;
1326 
1327     // Reentry phase -- reacquire the monitor.
1328     // re-enter contended monitor after object.wait().
1329     // retain OBJECT_WAIT state until re-enter successfully completes
1330     // Thread state is thread_in_vm and oop access is again safe,
1331     // although the raw address of the object may have changed.
1332     // (Don't cache naked oops over safepoints, of course).
1333 
1334     // post monitor waited event. Note that this is past-tense, we are done waiting.
1335     if (JvmtiExport::should_post_monitor_waited()) {
1336       JvmtiExport::post_monitor_waited(jt, this, ret == OS_TIMEOUT);
1337 
1338       if (node._notified != 0 && _succ == Self) {
1339         // In this part of the monitor wait-notify-reenter protocol it
1340         // is possible (and normal) for another thread to do a fastpath
1341         // monitor enter-exit while this thread is still trying to get
1342         // to the reenter portion of the protocol.
1343         //
1344         // The ObjectMonitor was notified and the current thread is
1345         // the successor which also means that an unpark() has already
1346         // been done. The JVMTI_EVENT_MONITOR_WAITED event handler can
1347         // consume the unpark() that was done when the successor was
1348         // set because the same ParkEvent is shared between Java
1349         // monitors and JVM/TI RawMonitors (for now).
1350         //
1351         // We redo the unpark() to ensure forward progress, i.e., we
1352         // don't want all pending threads hanging (parked) with none
1353         // entering the unlocked monitor.
1354         node._event->unpark();
1355       }
1356     }
1357 
1358     if (event.should_commit()) {
1359       post_monitor_wait_event(&event, this, node._notifier_tid, millis, ret == OS_TIMEOUT);
1360     }
1361 
1362     OrderAccess::fence();
1363 
1364     assert(Self->_Stalled != 0, "invariant");
1365     Self->_Stalled = 0;
1366 
1367     assert(_owner != Self, "invariant");
1368     ObjectWaiter::TStates v = node.TState;
1369     if (v == ObjectWaiter::TS_RUN) {
1370       enter(Self);
1371     } else {
1372       guarantee(v == ObjectWaiter::TS_ENTER || v == ObjectWaiter::TS_CXQ, "invariant");
1373       ReenterI(Self, &node);
1374       node.wait_reenter_end(this);
1375     }
1376 
1377     // Self has reacquired the lock.
1378     // Lifecycle - the node representing Self must not appear on any queues.
1379     // Node is about to go out-of-scope, but even if it were immortal we wouldn't
1380     // want residual elements associated with this thread left on any lists.
1381     guarantee(node.TState == ObjectWaiter::TS_RUN, "invariant");
1382     assert(_owner == Self, "invariant");
1383     assert(_succ != Self, "invariant");
1384   } // OSThreadWaitState()
1385 
1386   jt->set_current_waiting_monitor(NULL);
1387 
1388   guarantee(_recursions == 0, "invariant");
1389   _recursions = save;     // restore the old recursion count
1390   _waiters--;             // decrement the number of waiters
1391 
1392   // Verify a few postconditions
1393   assert(_owner == Self, "invariant");
1394   assert(_succ != Self, "invariant");
1395   assert(((oop)(object()))->mark() == markOopDesc::encode(this), "invariant");
1396 
1397   // check if the notification happened
1398   if (!WasNotified) {
1399     // no, it could be timeout or Thread.interrupt() or both
1400     // check for interrupt event, otherwise it is timeout
1401     if (interruptible && Thread::is_interrupted(Self, true) && !HAS_PENDING_EXCEPTION) {
1402       THROW(vmSymbols::java_lang_InterruptedException());
1403     }
1404   }
1405 
1406   // NOTE: Spurious wake up will be consider as timeout.
1407   // Monitor notify has precedence over thread interrupt.
1408 }
1409 
1410 
1411 // Consider:
1412 // If the lock is cool (cxq == null && succ == null) and we're on an MP system
1413 // then instead of transferring a thread from the WaitSet to the EntryList
1414 // we might just dequeue a thread from the WaitSet and directly unpark() it.
1415 
1416 void ObjectMonitor::INotify(Thread * Self) {


1417   Thread::SpinAcquire(&_WaitSetLock, "WaitSet - notify");
1418   ObjectWaiter * iterator = DequeueWaiter();
1419   if (iterator != NULL) {
1420     guarantee(iterator->TState == ObjectWaiter::TS_WAIT, "invariant");
1421     guarantee(iterator->_notified == 0, "invariant");
1422     // Disposition - what might we do with iterator ?
1423     // a.  add it directly to the EntryList - either tail (policy == 1)
1424     //     or head (policy == 0).
1425     // b.  push it onto the front of the _cxq (policy == 2).
1426     // For now we use (b).
1427 
1428     iterator->TState = ObjectWaiter::TS_ENTER;
1429 
1430     iterator->_notified = 1;
1431     iterator->_notifier_tid = JFR_THREAD_ID(Self);
1432 
1433     ObjectWaiter * list = _EntryList;
1434     if (list != NULL) {
1435       assert(list->_prev == NULL, "invariant");
1436       assert(list->TState == ObjectWaiter::TS_ENTER, "invariant");
1437       assert(list != iterator, "invariant");
1438     }
1439 
1440     // prepend to cxq

























1441     if (list == NULL) {
1442       iterator->_next = iterator->_prev = NULL;
1443       _EntryList = iterator;
1444     } else {
1445       iterator->TState = ObjectWaiter::TS_CXQ;
1446       for (;;) {
1447         ObjectWaiter * front = _cxq;
1448         iterator->_next = front;
1449         if (Atomic::cmpxchg(iterator, &_cxq, front) == front) {
1450           break;
1451         }
1452       }
1453     }























1454 
1455     // _WaitSetLock protects the wait queue, not the EntryList.  We could
1456     // move the add-to-EntryList operation, above, outside the critical section
1457     // protected by _WaitSetLock.  In practice that's not useful.  With the
1458     // exception of  wait() timeouts and interrupts the monitor owner
1459     // is the only thread that grabs _WaitSetLock.  There's almost no contention
1460     // on _WaitSetLock so it's not profitable to reduce the length of the
1461     // critical section.
1462 

1463     iterator->wait_reenter_begin(this);
1464   }

1465   Thread::SpinRelease(&_WaitSetLock);
1466 }
1467 
1468 // Consider: a not-uncommon synchronization bug is to use notify() when
1469 // notifyAll() is more appropriate, potentially resulting in stranded
1470 // threads; this is one example of a lost wakeup. A useful diagnostic
1471 // option is to force all notify() operations to behave as notifyAll().
1472 //
1473 // Note: We can also detect many such problems with a "minimum wait".
1474 // When the "minimum wait" is set to a small non-zero timeout value
1475 // and the program does not hang whereas it did absent "minimum wait",
1476 // that suggests a lost wakeup bug.
1477 
1478 void ObjectMonitor::notify(TRAPS) {
1479   CHECK_OWNER();
1480   if (_WaitSet == NULL) {
1481     return;
1482   }
1483   DTRACE_MONITOR_PROBE(notify, this, object(), THREAD);
1484   INotify(THREAD);
1485   OM_PERFDATA_OP(Notifications, inc(1));
1486 }
1487 
1488 
1489 // The current implementation of notifyAll() transfers the waiters one-at-a-time
1490 // from the waitset to the EntryList. This could be done more efficiently with a
1491 // single bulk transfer but in practice it's not time-critical. Beware too,
1492 // that in prepend-mode we invert the order of the waiters. Let's say that the
1493 // waitset is "ABCD" and the EntryList is "XYZ". After a notifyAll() in prepend
1494 // mode the waitset will be empty and the EntryList will be "DCBAXYZ".
1495 
1496 void ObjectMonitor::notifyAll(TRAPS) {
1497   CHECK_OWNER();
1498   if (_WaitSet == NULL) {
1499     return;
1500   }
1501 
1502   DTRACE_MONITOR_PROBE(notifyAll, this, object(), THREAD);
1503   int tally = 0;
1504   while (_WaitSet != NULL) {
1505     tally++;
1506     INotify(THREAD);
1507   }
1508 
1509   OM_PERFDATA_OP(Notifications, inc(tally));
1510 }
1511 
1512 // -----------------------------------------------------------------------------
1513 // Adaptive Spinning Support
1514 //
1515 // Adaptive spin-then-block - rational spinning
1516 //
1517 // Note that we spin "globally" on _owner with a classic SMP-polite TATAS
1518 // algorithm.  On high order SMP systems it would be better to start with
1519 // a brief global spin and then revert to spinning locally.  In the spirit of MCS/CLH,
1520 // a contending thread could enqueue itself on the cxq and then spin locally
1521 // on a thread-specific variable such as its ParkEvent._Event flag.
1522 // That's left as an exercise for the reader.  Note that global spinning is
1523 // not problematic on Niagara, as the L2 cache serves the interconnect and
1524 // has both low latency and massive bandwidth.
1525 //
1526 // Broadly, we can fix the spin frequency -- that is, the % of contended lock
1527 // acquisition attempts where we opt to spin --  at 100% and vary the spin count
1528 // (duration) or we can fix the count at approximately the duration of
1529 // a context switch and vary the frequency.   Of course we could also
1530 // vary both satisfying K == Frequency * Duration, where K is adaptive by monitor.
1531 // For a description of 'Adaptive spin-then-block mutual exclusion in
1532 // multi-threaded processing,' see U.S. Pat. No. 8046758.
1533 //
1534 // This implementation varies the duration "D", where D varies with
1535 // the success rate of recent spin attempts. (D is capped at approximately
1536 // length of a round-trip context switch).  The success rate for recent
1537 // spin attempts is a good predictor of the success rate of future spin
1538 // attempts.  The mechanism adapts automatically to varying critical
1539 // section length (lock modality), system load and degree of parallelism.
1540 // D is maintained per-monitor in _SpinDuration and is initialized
1541 // optimistically.  Spin frequency is fixed at 100%.
1542 //
1543 // Note that _SpinDuration is volatile, but we update it without locks
1544 // or atomics.  The code is designed so that _SpinDuration stays within
1545 // a reasonable range even in the presence of races.  The arithmetic
1546 // operations on _SpinDuration are closed over the domain of legal values,
1547 // so at worst a race will install and older but still legal value.
1548 // At the very worst this introduces some apparent non-determinism.
1549 // We might spin when we shouldn't or vice-versa, but since the spin
1550 // count are relatively short, even in the worst case, the effect is harmless.
1551 //
1552 // Care must be taken that a low "D" value does not become an
1553 // an absorbing state.  Transient spinning failures -- when spinning
1554 // is overall profitable -- should not cause the system to converge
1555 // on low "D" values.  We want spinning to be stable and predictable
1556 // and fairly responsive to change and at the same time we don't want
1557 // it to oscillate, become metastable, be "too" non-deterministic,
1558 // or converge on or enter undesirable stable absorbing states.
1559 //
1560 // We implement a feedback-based control system -- using past behavior
1561 // to predict future behavior.  We face two issues: (a) if the
1562 // input signal is random then the spin predictor won't provide optimal
1563 // results, and (b) if the signal frequency is too high then the control
1564 // system, which has some natural response lag, will "chase" the signal.
1565 // (b) can arise from multimodal lock hold times.  Transient preemption
1566 // can also result in apparent bimodal lock hold times.
1567 // Although sub-optimal, neither condition is particularly harmful, as
1568 // in the worst-case we'll spin when we shouldn't or vice-versa.
1569 // The maximum spin duration is rather short so the failure modes aren't bad.
1570 // To be conservative, I've tuned the gain in system to bias toward
1571 // _not spinning.  Relatedly, the system can sometimes enter a mode where it
1572 // "rings" or oscillates between spinning and not spinning.  This happens
1573 // when spinning is just on the cusp of profitability, however, so the
1574 // situation is not dire.  The state is benign -- there's no need to add
1575 // hysteresis control to damp the transition rate between spinning and
1576 // not spinning.
1577 
1578 // Spinning: Fixed frequency (100%), vary duration
1579 int ObjectMonitor::TrySpin(Thread * Self) {
1580   // Dumb, brutal spin.  Good for comparative measurements against adaptive spinning.
1581   int ctr = Knob_FixedSpin;
1582   if (ctr != 0) {
1583     while (--ctr >= 0) {
1584       if (TryLock(Self) > 0) return 1;
1585       SpinPause();
1586     }
1587     return 0;
1588   }
1589 
1590   for (ctr = Knob_PreSpin + 1; --ctr >= 0;) {
1591     if (TryLock(Self) > 0) {
1592       // Increase _SpinDuration ...
1593       // Note that we don't clamp SpinDuration precisely at SpinLimit.
1594       // Raising _SpurDuration to the poverty line is key.
1595       int x = _SpinDuration;
1596       if (x < Knob_SpinLimit) {
1597         if (x < Knob_Poverty) x = Knob_Poverty;
1598         _SpinDuration = x + Knob_BonusB;
1599       }
1600       return 1;
1601     }
1602     SpinPause();
1603   }
1604 
1605   // Admission control - verify preconditions for spinning
1606   //
1607   // We always spin a little bit, just to prevent _SpinDuration == 0 from
1608   // becoming an absorbing state.  Put another way, we spin briefly to
1609   // sample, just in case the system load, parallelism, contention, or lock
1610   // modality changed.
1611   //
1612   // Consider the following alternative:
1613   // Periodically set _SpinDuration = _SpinLimit and try a long/full
1614   // spin attempt.  "Periodically" might mean after a tally of
1615   // the # of failed spin attempts (or iterations) reaches some threshold.
1616   // This takes us into the realm of 1-out-of-N spinning, where we
1617   // hold the duration constant but vary the frequency.
1618 
1619   ctr = _SpinDuration;

1620   if (ctr <= 0) return 0;
1621 
1622   if (NotRunnable(Self, (Thread *) _owner)) {







1623     return 0;
1624   }



1625 
1626   // We're good to spin ... spin ingress.
1627   // CONSIDER: use Prefetch::write() to avoid RTS->RTO upgrades
1628   // when preparing to LD...CAS _owner, etc and the CAS is likely
1629   // to succeed.
1630   if (_succ == NULL) {
1631     _succ = Self;
1632   }



1633   Thread * prv = NULL;
1634 
1635   // There are three ways to exit the following loop:
1636   // 1.  A successful spin where this thread has acquired the lock.
1637   // 2.  Spin failure with prejudice
1638   // 3.  Spin failure without prejudice
1639 
1640   while (--ctr >= 0) {
1641 
1642     // Periodic polling -- Check for pending GC
1643     // Threads may spin while they're unsafe.
1644     // We don't want spinning threads to delay the JVM from reaching
1645     // a stop-the-world safepoint or to steal cycles from GC.
1646     // If we detect a pending safepoint we abort in order that
1647     // (a) this thread, if unsafe, doesn't delay the safepoint, and (b)
1648     // this thread, if safe, doesn't steal cycles from GC.
1649     // This is in keeping with the "no loitering in runtime" rule.
1650     // We periodically check to see if there's a safepoint pending.
1651     if ((ctr & 0xFF) == 0) {
1652       if (SafepointMechanism::poll(Self)) {
1653         goto Abort;           // abrupt spin egress
1654       }
1655       SpinPause();

























1656     }
1657 
1658     // Probe _owner with TATAS
1659     // If this thread observes the monitor transition or flicker
1660     // from locked to unlocked to locked, then the odds that this
1661     // thread will acquire the lock in this spin attempt go down
1662     // considerably.  The same argument applies if the CAS fails
1663     // or if we observe _owner change from one non-null value to
1664     // another non-null value.   In such cases we might abort
1665     // the spin without prejudice or apply a "penalty" to the
1666     // spin count-down variable "ctr", reducing it by 100, say.
1667 
1668     Thread * ox = (Thread *) _owner;
1669     if (ox == NULL) {
1670       ox = (Thread*)Atomic::cmpxchg(Self, &_owner, (void*)NULL);
1671       if (ox == NULL) {
1672         // The CAS succeeded -- this thread acquired ownership
1673         // Take care of some bookkeeping to exit spin state.
1674         if (_succ == Self) {
1675           _succ = NULL;
1676         }

1677 
1678         // Increase _SpinDuration :
1679         // The spin was successful (profitable) so we tend toward
1680         // longer spin attempts in the future.
1681         // CONSIDER: factor "ctr" into the _SpinDuration adjustment.
1682         // If we acquired the lock early in the spin cycle it
1683         // makes sense to increase _SpinDuration proportionally.
1684         // Note that we don't clamp SpinDuration precisely at SpinLimit.
1685         int x = _SpinDuration;
1686         if (x < Knob_SpinLimit) {
1687           if (x < Knob_Poverty) x = Knob_Poverty;
1688           _SpinDuration = x + Knob_Bonus;
1689         }
1690         return 1;
1691       }
1692 
1693       // The CAS failed ... we can take any of the following actions:
1694       // * penalize: ctr -= CASPenalty
1695       // * exit spin with prejudice -- goto Abort;
1696       // * exit spin without prejudice.
1697       // * Since CAS is high-latency, retry again immediately.
1698       prv = ox;
1699       goto Abort;



1700     }
1701 
1702     // Did lock ownership change hands ?
1703     if (ox != prv && prv != NULL) {
1704       goto Abort;


1705     }
1706     prv = ox;
1707 
1708     // Abort the spin if the owner is not executing.
1709     // The owner must be executing in order to drop the lock.
1710     // Spinning while the owner is OFFPROC is idiocy.
1711     // Consider: ctr -= RunnablePenalty ;
1712     if (NotRunnable(Self, ox)) {
1713       goto Abort;
1714     }
1715     if (_succ == NULL) {
1716       _succ = Self;
1717     }
1718   }
1719 
1720   // Spin failed with prejudice -- reduce _SpinDuration.
1721   // TODO: Use an AIMD-like policy to adjust _SpinDuration.
1722   // AIMD is globally stable.
1723   {
1724     int x = _SpinDuration;
1725     if (x > 0) {
1726       // Consider an AIMD scheme like: x -= (x >> 3) + 100
1727       // This is globally sample and tends to damp the response.
1728       x -= Knob_Penalty;
1729       if (x < 0) x = 0;
1730       _SpinDuration = x;
1731     }
1732   }
1733 
1734  Abort:
1735   if (_succ == Self) {

1736     _succ = NULL;
1737     // Invariant: after setting succ=null a contending thread
1738     // must recheck-retry _owner before parking.  This usually happens
1739     // in the normal usage of TrySpin(), but it's safest
1740     // to make TrySpin() as foolproof as possible.
1741     OrderAccess::fence();
1742     if (TryLock(Self) > 0) return 1;
1743   }
1744   return 0;
1745 }
1746 
1747 // NotRunnable() -- informed spinning
1748 //
1749 // Don't bother spinning if the owner is not eligible to drop the lock.
1750 // Spin only if the owner thread is _thread_in_Java or _thread_in_vm.
1751 // The thread must be runnable in order to drop the lock in timely fashion.
1752 // If the _owner is not runnable then spinning will not likely be
1753 // successful (profitable).
1754 //
1755 // Beware -- the thread referenced by _owner could have died
1756 // so a simply fetch from _owner->_thread_state might trap.
1757 // Instead, we use SafeFetchXX() to safely LD _owner->_thread_state.
1758 // Because of the lifecycle issues, the _thread_state values
1759 // observed by NotRunnable() might be garbage.  NotRunnable must
1760 // tolerate this and consider the observed _thread_state value
1761 // as advisory.
1762 //
1763 // Beware too, that _owner is sometimes a BasicLock address and sometimes
1764 // a thread pointer.
1765 // Alternately, we might tag the type (thread pointer vs basiclock pointer)
1766 // with the LSB of _owner.  Another option would be to probabilistically probe
1767 // the putative _owner->TypeTag value.
1768 //
1769 // Checking _thread_state isn't perfect.  Even if the thread is
1770 // in_java it might be blocked on a page-fault or have been preempted
1771 // and sitting on a ready/dispatch queue.
1772 //
1773 // The return value from NotRunnable() is *advisory* -- the
1774 // result is based on sampling and is not necessarily coherent.
1775 // The caller must tolerate false-negative and false-positive errors.
1776 // Spinning, in general, is probabilistic anyway.
1777 
1778 
1779 int ObjectMonitor::NotRunnable(Thread * Self, Thread * ox) {
1780   // Check ox->TypeTag == 2BAD.
1781   if (ox == NULL) return 0;
1782 
1783   // Avoid transitive spinning ...
1784   // Say T1 spins or blocks trying to acquire L.  T1._Stalled is set to L.
1785   // Immediately after T1 acquires L it's possible that T2, also
1786   // spinning on L, will see L.Owner=T1 and T1._Stalled=L.
1787   // This occurs transiently after T1 acquired L but before
1788   // T1 managed to clear T1.Stalled.  T2 does not need to abort
1789   // its spin in this circumstance.
1790   intptr_t BlockedOn = SafeFetchN((intptr_t *) &ox->_Stalled, intptr_t(1));
1791 
1792   if (BlockedOn == 1) return 1;
1793   if (BlockedOn != 0) {
1794     return BlockedOn != intptr_t(this) && _owner == ox;
1795   }
1796 
1797   assert(sizeof(((JavaThread *)ox)->_thread_state == sizeof(int)), "invariant");
1798   int jst = SafeFetch32((int *) &((JavaThread *) ox)->_thread_state, -1);;
1799   // consider also: jst != _thread_in_Java -- but that's overspecific.
1800   return jst == _thread_blocked || jst == _thread_in_native;
1801 }
1802 
1803 
1804 // -----------------------------------------------------------------------------
1805 // WaitSet management ...
1806 
1807 ObjectWaiter::ObjectWaiter(Thread* thread) {
1808   _next     = NULL;
1809   _prev     = NULL;
1810   _notified = 0;
1811   _notifier_tid = 0;
1812   TState    = TS_RUN;
1813   _thread   = thread;
1814   _event    = thread->_ParkEvent;
1815   _active   = false;
1816   assert(_event != NULL, "invariant");
1817 }
1818 
1819 void ObjectWaiter::wait_reenter_begin(ObjectMonitor * const mon) {
1820   JavaThread *jt = (JavaThread *)this->_thread;
1821   _active = JavaThreadBlockedOnMonitorEnterState::wait_reenter_begin(jt, mon);
1822 }
1823 
1824 void ObjectWaiter::wait_reenter_end(ObjectMonitor * const mon) {
1825   JavaThread *jt = (JavaThread *)this->_thread;
1826   JavaThreadBlockedOnMonitorEnterState::wait_reenter_end(jt, _active);
1827 }
1828 
1829 inline void ObjectMonitor::AddWaiter(ObjectWaiter* node) {
1830   assert(node != NULL, "should not add NULL node");
1831   assert(node->_prev == NULL, "node already in list");
1832   assert(node->_next == NULL, "node already in list");
1833   // put node at end of queue (circular doubly linked list)
1834   if (_WaitSet == NULL) {
1835     _WaitSet = node;
1836     node->_prev = node;
1837     node->_next = node;
1838   } else {
1839     ObjectWaiter* head = _WaitSet;
1840     ObjectWaiter* tail = head->_prev;
1841     assert(tail->_next == head, "invariant check");
1842     tail->_next = node;
1843     head->_prev = node;
1844     node->_next = head;
1845     node->_prev = tail;
1846   }
1847 }
1848 
1849 inline ObjectWaiter* ObjectMonitor::DequeueWaiter() {
1850   // dequeue the very first waiter
1851   ObjectWaiter* waiter = _WaitSet;
1852   if (waiter) {
1853     DequeueSpecificWaiter(waiter);
1854   }
1855   return waiter;
1856 }
1857 
1858 inline void ObjectMonitor::DequeueSpecificWaiter(ObjectWaiter* node) {
1859   assert(node != NULL, "should not dequeue NULL node");
1860   assert(node->_prev != NULL, "node already removed from list");
1861   assert(node->_next != NULL, "node already removed from list");
1862   // when the waiter has woken up because of interrupt,
1863   // timeout or other spurious wake-up, dequeue the
1864   // waiter from waiting list
1865   ObjectWaiter* next = node->_next;
1866   if (next == node) {
1867     assert(node->_prev == node, "invariant check");
1868     _WaitSet = NULL;
1869   } else {
1870     ObjectWaiter* prev = node->_prev;
1871     assert(prev->_next == node, "invariant check");
1872     assert(next->_prev == node, "invariant check");
1873     next->_prev = prev;
1874     prev->_next = next;
1875     if (_WaitSet == node) {
1876       _WaitSet = next;
1877     }
1878   }
1879   node->_next = NULL;
1880   node->_prev = NULL;
1881 }
1882 
1883 // -----------------------------------------------------------------------------
1884 // PerfData support
1885 PerfCounter * ObjectMonitor::_sync_ContendedLockAttempts       = NULL;
1886 PerfCounter * ObjectMonitor::_sync_FutileWakeups               = NULL;
1887 PerfCounter * ObjectMonitor::_sync_Parks                       = NULL;
1888 PerfCounter * ObjectMonitor::_sync_Notifications               = NULL;
1889 PerfCounter * ObjectMonitor::_sync_Inflations                  = NULL;
1890 PerfCounter * ObjectMonitor::_sync_Deflations                  = NULL;
1891 PerfLongVariable * ObjectMonitor::_sync_MonExtant              = NULL;
1892 
1893 // One-shot global initialization for the sync subsystem.
1894 // We could also defer initialization and initialize on-demand
1895 // the first time we call inflate().  Initialization would
1896 // be protected - like so many things - by the MonitorCache_lock.
1897 
1898 void ObjectMonitor::Initialize() {
1899   static int InitializationCompleted = 0;
1900   assert(InitializationCompleted == 0, "invariant");
1901   InitializationCompleted = 1;
1902   if (UsePerfData) {
1903     EXCEPTION_MARK;
1904 #define NEWPERFCOUNTER(n)                                                \
1905   {                                                                      \
1906     n = PerfDataManager::create_counter(SUN_RT, #n, PerfData::U_Events,  \
1907                                         CHECK);                          \
1908   }
1909 #define NEWPERFVARIABLE(n)                                                \
1910   {                                                                       \
1911     n = PerfDataManager::create_variable(SUN_RT, #n, PerfData::U_Events,  \
1912                                          CHECK);                          \
1913   }
1914     NEWPERFCOUNTER(_sync_Inflations);
1915     NEWPERFCOUNTER(_sync_Deflations);
1916     NEWPERFCOUNTER(_sync_ContendedLockAttempts);
1917     NEWPERFCOUNTER(_sync_FutileWakeups);
1918     NEWPERFCOUNTER(_sync_Parks);
1919     NEWPERFCOUNTER(_sync_Notifications);
1920     NEWPERFVARIABLE(_sync_MonExtant);
1921 #undef NEWPERFCOUNTER
1922 #undef NEWPERFVARIABLE
1923   }
1924 }
1925 























1926 void ObjectMonitor::DeferredInitialize() {
1927   if (InitDone > 0) return;
1928   if (Atomic::cmpxchg (-1, &InitDone, 0) != 0) {
1929     while (InitDone != 1) /* empty */;
1930     return;
1931   }
1932 
1933   // One-shot global initialization ...
1934   // The initialization is idempotent, so we don't need locks.
1935   // In the future consider doing this via os::init_2().




1936 
1937   if (!os::is_MP()) {



















































1938     Knob_SpinLimit = 0;

1939     Knob_PreSpin   = 0;
1940     Knob_FixedSpin = -1;
1941   }
1942 

1943   OrderAccess::fence();
1944   InitDone = 1;
1945 }
1946 
--- EOF ---